Offset printing is a printing method in which ink adhering to a printing plate surface is not directly transferred to paper, but the ink is once transferred to a rubber blanket or the like and then transferred to a material to be printed such as paper. Although there are various methods for making offset printing plates, they can be roughly classified into analog plate-making methods using PS plates and direct plate-making methods using a comparatively simple DTP technique, as the most common methods.
The plate-making methods using PS plates constitute the mainstream of the conventional methods, in which a photographic film of an original is prepared beforehand, and then superimposed on a PS plate comprising an aluminum substrate on which a photosensitive resin is laminated, and light exposure (printing) and dissolution of unexposed portions (development) are performed to obtain the aluminum plate on which exposed portion remain as a printing plate.
On the other hand, the direct plate-making methods are methods of directly forming original images on a substrate such as plastic or aluminum plates without using a block copy film as in the conventional methods, and electrophotographic methods and silver photographic methods have conventionally been used in many cases. However, methods based on the ink-jet method, thermal transfer method, or discharge transfer method are also practically used.
Among the direct plate-making methods, those using the ink-jet method are techniques existing from old time and they are methods of scanning a surface of a support having an image receiving layer with a printing means based on the ink-jet method discharging droplets of lipophilic ink to form images of a printing plate.
According to the methods of this type, a printing plate can be directly made without a procedure of once printing out electronized information of an original as a hard copy, and they further have convenience that, for example, the information of the original can be stored in a storage medium, and freely edited on a personal computer or the like.
Patent Reference 1 suggests a technique which seems to relate to the present invention.    [Patent Reference 1] Japanese Patent Application Publication No. 2000-037937